Duplexer and Method for Increasing the Isolation Between Two Filters

ABSTRACT

A duplexer includes an antenna terminal, a transmission amplifier terminal and a reception amplifier terminal. The transmission amplifier terminal is coupled to the antenna terminal via a transmission filter. The reception amplifier terminal is coupled to a reception filter and the reception filter is coupled to the antenna terminal via a band-stop filter.

This application is a continuation of co-pending InternationalApplication No. PCT/EP2009/060841, filed Aug. 21, 2009, which designatedthe United States and was not published in English, and which claimspriority to German Application No. 10 2008 045 346.3, filed Sep. 1,2008, both of which applications are incorporated herein by reference.

BACKGROUND

The invention relates to a duplexer in which, in the transmission path,a transmission signal is led from a transmission amplifier to theantenna and, in the reception path, a reception signal is led from theantenna to a reception amplifier. In order to be able to transmit andreceive simultaneously, different frequency ranges are used for thetransmission signal and the reception signal. In order that thetransmission signal does not interfere with the very much weakerreception signal, a reception filter is provided in the reception path,which reception filter passes the reception signal and greatlysuppresses the transmission signal. An isolation of 50 dB to 60 dBbetween the reception path and the transmission path is typicallyrequired in the transmission frequency band.

In the case of closely adjacent transmission and reception frequencyranges, the isolation in the transmission frequency range is determinedby the selection of the reception filter, which is usually embodied as abandpass filter. If the reception frequency range lies above thetransmission frequency range, the selection is predetermined by thegradient of the lower reception filter edges. With a predeterminedreception filter bandwidth and the required impedance matching, however,the selection cannot be increased arbitrarily owing to the dictates ofdesign. With the aid of inductances, an improvement in the isolation ofapproximately 10 dB can be achieved by means of a pole shift. However,this method can be used only to a limited extent in the case ofduplexers having a small separation between the transmission andreception frequency bands of only approximately 20 MHz and attransmission and reception frequencies of approximately 1.9 GHz, sincethe lower reception filter edge is flattened. Furthermore, theinductance values have fluctuations that reduce the transmission rangeisolation specified as typical.

SUMMARY

In one aspect the invention increases the isolation between thetransmission and reception paths at transmission frequencies.

In one embodiment the invention provides a duplexer comprising anantenna terminal, a transmission amplifier terminal and a receptionamplifier terminal. The transmission amplifier terminal is coupled tothe antenna terminal via a transmission filter and the receptionamplifier terminal is coupled to a reception filter. The receptionfilter is coupled to the antenna terminal via a band-stop filter. Theband-stop filter separates the transmission path from the receptionpath, such that the isolation of the duplexer is increased.

The transmission filter has a passband, while the band-stop filter has astop band in the passband of the transmission filter. Since the stopband of the band-stop filter lies in the passband of the transmissionfilter, the isolation in the transmission frequency range is increased.

The transmission filter comprises at least one resonator and theband-stop filter comprises at least one same resonator as thetransmission filter. In the application, the term “same resonators” istaken to mean resonators which have the same resonant frequencies andthe same layer construction with regard to materials and layerthicknesses. However, they can have different areas and thus havedifferent static capacitances. Since the band-stop filter and thetransmission filter have the same resonator, the design outlay and themanufacture of the duplexer can be simplified.

In one development, the transmission filter and the band-stop filtercomprise ladder structures having series resonators or parallelresonators. The desired transfer properties of the transmission filterand of the band-stop filter can be synthesized with the aid of cascadedladder structures.

In one development, at least one parallel resonator of the transmissionfilter has a resonant frequency that is lower than the resonantfrequencies of the series resonators. The bandwidth of the transmissionfilter and of the band-stop filter can be set by means of the differentresonant frequencies.

In one development, at least one series resonator of the band-stopfilter is the same resonator as a parallel resonator of the transmissionfilter. The same manufacturing process used for the parallel resonatorof the transmission filter can therefore also be used for the seriesresonator of the band-stop filter. Furthermore, the antiresonance of theparallel resonator then lies in series with the reception filter andincreases the isolation.

In one development, at least one parallel resonator of the band-stopfilter is the same resonator as a series resonator of the transmissionfilter. In this case, too, both the parallel resonator of the band-stopfilter and the series resonator of the transmission filter can berealized in one process. By virtue of the parallel resonator and theseries resonator of the transmission filter being used as a seriesresonator and parallel resonator, respectively, of the band-stop filter,the band-stop filter acquires a transfer function that is quasi inversewith respect to the passband of the transmission filter.

In one development, the resonant frequency of at least one seriesresonator of the band-stop filter differs from the resonant frequenciesof the remaining series resonators of the band-stop filter.

In one development, the resonant frequency of at least one parallelresonator of the band-stop filter differs from the resonant frequenciesof the remaining parallel resonators of the band-stop filter. Thedifferent resonant frequencies of the parallel and series resonatorsresult in additional degrees of freedom being available which can beused to optimize the duplexer with regard to the matching and thetransfer function and the isolation.

In one development, the resonators whose resonant frequencies differfrom the other resonators have an additional mass coating which changesthe resonant frequency and which is not present, or not present to thesame extent, in the other resonators. If the additional mass coating canbe applied using simple means, then it constitutes one possibility ofinfluencing the resonant frequency without great outlay and withoutcomplicated process steps.

In one development, the additional mass coating is a titanium layer, analuminum layer, a molybdenum layer, an iridium layer, a ruthenium layer,a silicon nitride layer, an aluminum nitride layer, a zinc oxide layer,a lead zirconate titanate (PZT) layer, a barium strontium titanate (BST)layer, or a layer composed of some other material, which layer isapplied above or below a piezo-layer of the resonator, or thickens thepiezo-layer. These layers can be applied in a targeted manner, or can beremoved by simple process steps, such that it is possible to change theresonant frequency without great outlay.

In one development, the resonators of the transmission filter and theresonators of the band-stop filter are BAW resonators and are arrangedon the same substrate. The resonators of the transmission filter and ofthe band-stop filter can, if they are the same in accordance with theabove definition, be manufactured by means of the same process, as aresult of which the number of process steps is reduced.

In one development, the resonators of the reception filter and theresonators of the band-stop filter are BAW (Bulk Acoustic Wave)resonators and are arranged on the same substrate.

In one development, the resonators of the transmission filter, theresonators of the reception filter, and the resonators of the band-stopfilter are BAW resonators and are arranged on the same substrate.

In one development, the reception filter comprises at least one SAW(Surface Acoustic Wave) resonator. SAW resonators permitbalanced-unbalanced driving of the filters. SAW technology makes itpossible to realize greatly different frequencies on one substrate.

In one development, the transmission filter and the band-stop filtercomprise SAW resonators instead of BAW resonators, and the SAWresonators of the reception filter, of the transmission filter and ofthe band-stop filter are constructed on a common substrate.

SAW resonators having different resonant frequencies can be manufacturedtogether by means of the same process, as a result of which theproduction of the duplexer is simplified.

In one development, GBAW resonators are used instead of BAW resonators.GBAW (Guided Bulk Acoustic Waves) are SAW-like components in which theacoustic wave runs just below the component surface. The resonantfrequency of a GBAW resonator results firstly from the period of thefinger arrangement as in the case of the SAW resonator and secondly fromthe layer construction as in the case of the BAW resonator. Therefore,the methods for frequency reduction in the case of BAW resonators canalso be applied to GBAW resonators.

It is also possible for a duplexer to comprise both BAW and GBAWresonators and thus to be embodied as a hybrid duplexer.

The reception filter can comprise at least one GBAW resonator. Likewise,the transmission filter and the band-stop filter can comprise GBAWresonators instead of BAW resonators, wherein all the GBAW resonators ofthe reception filter, of the transmission filter and of the band-stopfilter can be constructed on a common substrate.

In one development, at least one of the resonators of the band-stopfilter has a first resonant frequency and a first static capacitance. Itcomprises a number N of resonators, wherein the number N is greater thanor equal to two, the N resonators each have a static capacitance that isN times greater than the first static capacitance, the N resonators areconnected in series with one another, and the resonant frequencydeviates from the first resonant frequency by up to 3% at least in oneof the N resonators. The multiplication of the resonator results infurther degrees of freedom for the filter design by virtue of multiplepoles being generated by the frequency deviation.

In one development, at least one of the resonators of the band-stopfilter has a first resonant frequency and a first static capacitance. Itcomprises a number N of resonators, wherein the number N is greater thanor equal to two, the N resonators each have a static capacitance that isN times less than the first static capacitance, the N resonators areconnected in parallel with one another, and the resonant frequencydeviates from the first resonant frequency by up to 3% at least in oneof the N resonators. The multiplication of the resonator results infurther degrees of freedom for the filter design by virtue of multiplepoles being generated by the frequency deviation.

In one development, the duplexer further comprises at least one matchingcircuit designed such that upon reception of a reception signal thetransmission filter constitutes an open circuit from the point of viewof the reception filter, and reflections between the reception filterand the antenna terminal are minimized, and upon transmission of atransmission signal the reception filter constitutes an open circuitfrom the point of view of the transmission filter, and reflectionsbetween the transmission filter and the antenna terminal are minimized.The transmission filter, the reception filter and the band-stop filterhave to be matched to one another and to the antenna terminal, such thatpower can be transferred with low losses and with the necessaryisolation between the terminals.

In one development, the band-stop filter has a first terminal and asecond terminal, wherein the first terminal is connected to the antennaterminal and the transmission filter and the second terminal isconnected to the reception filter. The matching circuit comprises afirst inductance and a second inductance, wherein the first inductanceconnects the first terminal to ground and the second inductance connectsthe second terminal to ground. The first inductance enables the matchingof the transmission filter, while the first inductance together with thesecond inductance and the static capacitance of the band-stop filterforms a PI matching network for the reception filter.

In one development, the band-stop filter comprises a first seriesresonator, which is connected to the first terminal. By means of thefirst series resonator, the transfer properties with respect to thereception filter can be influenced in a targeted manner.

In one development, the first series resonator has an antiresonancelying in the passband of the transmission filter. At the antiresonance,the first series resonator has a high impedance, such that a highisolation with respect to the reception filter arises at frequencies inthe passband of the transmission filter.

In one development, the band-stop filter further comprises at least oneparallel resonator, wherein a first terminal of the at least oneparallel resonator is connected to ground, and the other terminal of theat least one parallel resonator is connected to the second terminal andthe first series resonator. The bandwidth of the band-stop filter can beset by means of the parallel resonator.

In one development, at least one of the first terminals of the at leastone parallel resonator is connected to ground not directly but rathervia an inductance, or a capacitance, or a combination of an inductanceand a capacitance. The additional inductances enable further degrees offreedom in the matching of the duplexer.

In one development, the first series resonator has an antiresonance inthe region of the lower passband edge of the transmission filter and theparallel resonator has a resonant frequency lying in the region of thecenter of the passband of the transmission filter. Since theantiresonance lies in the region of the lower passband edge of thetransmission filter, the resonant frequency of the series resonator liesbelow the lower transmission filter passband edge and does not adverselyaffect the latter. The resonance of the parallel resonator in the regionof the center of the passband of the transmission filter ensures thattransmission frequencies are dissipated to ground and thus increase theisolation.

In one development, the first series resonator is the same as a parallelresonator of the transmission filter, but has an additional mass coatingwhich reduces the resonant frequency of the first series resonatorrelative to the resonant frequency of the parallel resonator, and theparallel resonator connected to the second terminal is the same as aseries resonator of the transmission filter. Since, in the production ofresonators using BAW technology, usually only two resonant frequenciesare available on one chip, the use of the same resonators both for theband-stop filter and for the transmission filter is advantageous. Thereduction of the resonant frequency by means of an additional masscoating results in further degrees of freedom in the design. The masscoating can be embodied in the manner already mentioned further above.

In one development, the band-stop filter comprises a first terminal anda second terminal, and two series resonators connected in series via aconnecting node. One of the series is connected to the first terminaland the other is connected to the second terminal. The first terminal isconnected to the transmission filter and the second terminal isconnected to the reception filter. The matching circuit comprises afirst inductance and a second inductance, wherein the first inductanceconnects the first terminal to the antenna terminal and the secondinductance connects the connecting node to ground. The first inductancesupplies the necessary inductive character at the antenna terminal,while the second inductance together with the capacitances of the seriesresonators forms a T-network for the matching of the reception filter.

In one development, the first terminal is not connected to thetransmission filter, but rather to the antenna terminal, and the firstinductance is not connected to the antenna terminal, but rather to thetransmission filter. In this way it is possible to configure thematching of the transmission filter independently of the matching of thereception filter.

In one development, the series resonator connected to the first terminalhas an antiresonance in the region of the lower passband of thetransmission filter, and the series resonator connected to the secondterminal has an antiresonance frequency lying in the region of thecenter of the passband of the transmission filter. The combination ofthe antiresonances leads to a further increase in the isolation betweenthe transmission and reception filters.

In one development, the series resonator connected to the first terminalis the same as a parallel resonator of the transmission filter, but hasan additional mass coating which reduces the resonant frequency of theseries resonator relative to the resonant frequency of the parallelresonator, and the series resonator connected to the second terminal isthe same as a parallel resonator without additional mass coating of thetransmission filter. The reduction of the resonant frequency has theeffect that the latter is no longer directly at the lower transmissionfilter passband side, as a result of which the latter is not adverselyaffected. The use of the same resonators for the band-stop filter andthe transmission filter makes it possible to produce these by means ofthe same process steps.

In one development, the first series resonator has an antiresonance buthas no resonance. For isolation purposes it suffices if the first seriesresonator has a very high impedance at the antiresonance frequency.

In one development, at least one parallel resonator has a resonance buthas no antiresonance. By means of the resonance, the resonators conductwell. For isolation purposes it suffices if a parallel resonator has alow impedance at the resonance.

In one development, the transmission filter comprises a series resonatorvia which it is connected to the first terminal. By virtue of the seriesresonator, the transmission filter at the antenna port in the receptionfrequency range behaves like an open circuit if it has an antiresonancein this frequency range.

In one development, the matching circuit further comprises an inductancewhich connects the transmission filter to the transmission amplifierterminal, and comprises an inductance which connects the receptionfilter to the reception amplifier terminal. The inductances serve formatching the transmission filter and the reception filter to therespective terminals.

In one development, further terminals are provided, which are coupled tothe antenna terminal via respective filters and band-stop filters,wherein the stop bands of the respective band-stop filters lie in thepassband of the transmission filter. The band-stop filters make itpossible to achieve a high isolation between a plurality of signalpaths. In addition, it is possible to minimize capacitive losses thatarise if the separation between the passbands from one filter to theother is sufficiently large and one filter thus acts as capacitiveloading on the other filter.

The invention furthermore provides a method for increasing the isolationbetween a first bandpass filter and at least one second bandpass filter,wherein the first bandpass filter and the at least one second bandpassfilter are coupled to a common node. The at least one second band passfilter is coupled to the common node via a respective band-stop filter,wherein the respective band-stop filters effect suppression in apassband of the first bandpass filter. One advantage of this method isthat the capacitive loading of the respective second filter by theband-stop filter is greatly reduced. This property is advantageousparticularly in the case of filters in greatly different frequencyranges.

In one development, the filter function of the first bandpass filter andthat of the band-stop filters are realized in each case by at least onesame resonator.

In one development, the resonators of the first bandpass filter and theresonators of the at least one band-stop filter are realized on the samesubstrate.

In one development, the same resonators are realized by means of thesame process steps.

In one development, the resonant frequency of at least one resonator ofthe band-stop filters is lowered relative to the resonant frequency ofthe same resonator of the first bandpass filter.

In one development, the resonant frequency is lowered by an additionalmass coating applied on the resonator.

In one development, the resonators of the band-stop filters and of thefirst bandpass filter are BAW resonators.

In one development, the resonators of the band-stop filters are SAWresonators.

In one development, the resonators of the band-stop filters are GBAWresonators.

In one development, the first bandpass filter, the at least one secondbandpass filter and an antenna coupled to the common node areimpedance-matched to one another such that at frequencies lying in thepassband of the first bandpass filter, the reflection of power betweenthe first bandpass filter and the antenna is minimized, and the at leastone second bandpass filter constitutes an open circuit from the point ofview of the first bandpass filter, and at frequencies lying in each casein the passbands of the at least one second bandpass filter, thereflection of power between each second bandpass filter and the antennais minimized, and the first bandpass filter constitutes an open circuitin each case from the point of view of the respective second bandpassfilter.

In one development, the static capacitance of at least one resonator ofthe band-stop filters is varied for the matching.

In one development, at least one of the resonators of the band-stopfilters has a first resonant frequency and a first static capacitanceand is realized by at least a number N of resonators, wherein the numberN of resonators is greater than or equal to two, the N resonators eachhave a static capacitance that is N times greater than the first staticcapacitance, the N resonators are connected in series with one another,and the resonant frequency deviates from the first resonant frequency byup to 3% at least in one of the resonators.

In one development, at least one of the resonators of the band-stopfilters has a first resonant frequency and a first static capacitanceand is realized by at least a number N of resonators, wherein the numberN of resonators is greater than or equal to two, the N resonators eachhave a static capacitance that is N times less than the first staticcapacitance, the N resonators are connected in parallel with oneanother, and the resonant frequency deviates from the first resonantfrequency by up to 3% at least in one of the resonators.

In one development, the resonators of the band-stop filters are SAWresonators.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below on the basis of exemplary embodimentswith the aid of the figures, in which:

FIG. 1 shows an exemplary embodiment of a duplexer;

FIG. 2 shows exemplary transfer functions between an antenna terminaland a transmission amplifier terminal, and between an antenna terminaland a reception filter and respectively a reception filter with aband-stop filter;

FIG. 3 shows exemplary isolation profiles between a transmissionamplifier terminal and a reception amplifier terminal;

FIGS. 4A-4E, collectively FIG. 4, show exemplary embodiments of ladderstructures with series resonators and parallel resonators;

FIG. 5 shows exemplary embodiments of a transmission filter, of aband-stop filter and of a reception filter;

FIGS. 6A-6D, collectively FIG. 6, show exemplary embodiments in whichresonators comprise a plurality of resonators;

FIGS. 7 to 10 show exemplary embodiments of a band-stop filter with amatching circuit; and

FIG. 11 shows an exemplary embodiment of a multiband duplexer with aplurality of band-stop filters and bandpass filters.

The following list of reference symbols may be used in conjunction withthe drawings:

-   1, 2 Terminals-   fp Resonant frequency of the parallel resonator-   fs Resonant frequency of the series resonator-   A Connecting node-   ANT Antenna terminal-   BS Band-stop filter-   C0 Static capacitance of the BAW resonator-   CT Substrate of the transmission filter-   CS Substrate of the reception filter-   D Duplexer-   GND Ground-   I Isolation without band-stop filter-   IB Isolation with band-stop filter-   K Common node-   L1, L2, L3, L4 Inductances-   LNA, LNA1, LNA2 Reception amplifier terminals-   M1, M2 Matching circuits-   N Number of resonators-   PA, PA1, PA2 Transmission amplifier terminals-   P, P1, P2 Parallel resonator-   PB1 Parallel resonator of the band-stop filter-   PT1, PT2 Parallel resonator of the transmission filter-   PR1, PR2, PR3 Parallel resonator of the reception filter-   R Transfer function of reception filter-   RB Transfer function of reception filter with band-stop filter-   RX, RX1, RX2 Reception filter, second bandpass filters-   T Transfer function of transmission filter-   TP Passband of the transmission filter-   TX, TX1 Transmission filter, first bandpass filter-   TX2, TX3 Second bandpass filters-   S, S1, S2 Series resonator-   SB1, SB2 Series resonator of the band-stop filter-   ST1, ST2, ST3 Series resonator of the transmission filter-   SR1, SR2 Series resonator of the reception filter

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an exemplary embodiment of a duplexer D comprising atransmission amplifier terminal PA, a reception amplifier terminal LNAand an antenna terminal ANT. The transmission amplifier terminal PA iscoupled to a transmission filter TX, and the reception amplifierterminal LNA is coupled to a reception filter RX. Two matching circuitsM1 and M2 are provided, by means of which the transmission filter TX andthe reception filter RX are impedance-matched to the antenna terminalANT, such that no power is reflected when a transmission signal istransmitted to the antenna and when a reception signal is received bythe antenna. The matching circuits M1 and M2 are designed such that thereception filter RX constitutes an open circuit from the point of viewof the transmission filter TX at transmission frequencies and thetransmission filter TX also constitutes an open circuit from the pointof view of the reception filter RX at reception frequencies. Theisolation between the transmission path and the reception path isincreased by means of the band-stop filter BS situated in the receptionsignal path. Such a duplexer D can be used, e.g., during operation inWCDMA (Wideband Code Division Multiple Access) band II for thesimultaneous transmission and reception of signals.

FIG. 2 and FIG. 3 show exemplary transfer functions, with and without aband-stop filter BS, between the antenna terminal ANT, the transmissionamplifier terminal PA and the reception amplifier terminal LNA. Thecurve T in FIG. 2 shows the transfer function between the transmissionamplifier terminal PA and the antenna terminal ANT. The curve RB showsthe transfer function between the reception amplifier terminal LNA andthe antenna terminal ANT. The curve R shows the transfer functionbetween the reception amplifier terminal LNA and the antenna terminalANT, but for comparison purposes without a band-stop filter BS. Thetransfer functions shown are already matched.

For WCDMA band II, the passband TP of the transmission signals liesbetween 1850 and 1910 MHz and has a width of 60 MHz. The reception bandlies above the transmission band, the reception band likewise having awidth of 60 MHz and lying between 1930 and 1990 MHz. Transmission andreception bands therefore have a separation from one another of just 20MHz.

As shown by the comparison between the transfer functions R and RB, inthe transmission frequency range a better suppression of transmissionsignals takes place by means of the band-stop filter BS than without aband-stop filter. Furthermore, the transfer functions R and RB lievirtually completely on top of each other in the reception frequencyrange, which shows that the band-stop filter BS only slightly influencesthe transfer function. It is furthermore important that the gradient ofthe left passband edge of the reception filter RX is maintainedunchanged. The design of the band-stop filter BS is additionallyindependent of that of the transmission filter TX and of the receptionfilter RX. It therefore constitutes a new functional element which canbe used to simplify the filter design by means of further degrees offreedom.

FIG. 3 shows the isolation between the transmission amplifier terminalPA and the reception amplifier terminal LNA. The curve I shows theisolation without a band-stop filter BS, while the curve IB shows theisolation with the band-stop filter BS. An improvement in the isolationfrom 40 dB to 60 dB is evident in the transmission frequency range.

FIGS. 4A to 4E show exemplary embodiments of ladder structures that canbe used for the construction of the transmission filter TX, of theband-stop filter BS and of the reception filter RX. FIG. 4A shows aseries resonator S, and FIG. 4B shows a parallel resonator P. FIGS. 4Cand 4D show a combination of series resonators S and parallel resonatorsP. FIG. 4E shows a T-arrangement comprising two series resonators S1 andS2 and one parallel resonator P. The ladder structures can beinterconnected with one another in a chain circuit in order to obtainmulti-stage ladder structures. The number of stages and the selection ofthe ladder structures themselves are determined from the requirementsmade of the filter transconductance, filter bandwidth and insertionloss.

The ladder structures shown in FIG. 4 are “single ended-single ended”.An extension for the “balanced-balanced” case occurs by mirroring theexisting filter part at the ground rail and eliminating the access tothe original ground node. Lattice structures are also possible for the“balanced-balanced” case.

The resonators of the ladder structures in FIG. 4 can be resonatorswhich can be described by a Butterworth-van Dyke model. These are, e.g.,resonators using bulk acoustic wave (BAW) technology, surface acousticwave (SAW) technology, guided bulk acoustic wave (GBAW) technology,other microacoustic technologies, resonators composed of concentratednetwork elements such as inductances and capacitances or generallyelectromagnetically effective elements. These resonators exhibit aresonance, at which they conduct well, and an antiresonance, at whichthey conduct poorly. Beyond these resonances, the resonators typicallyexhibit capacitive behavior.

Resonators having only an antiresonance in each case are sufficient forthe series resonators of the band-stop filter. A resonance is notnecessary. One example thereof is a parallel resonant circuit composedof an inductance and a capacitance. Resonators having only a resonancein each case are sufficient for the parallel resonators of the band-stopfilter. An antiresonance is not necessary. One example thereof is atuning fork or a MEMS oscillator.

FIG. 5 shows exemplary embodiments of the transmission filter TX, theband-stop filter BS and the reception filter RX. In this case, thetransmission filter TX consists of a chain circuit of two ladderstructures in accordance with FIG. 4C and one ladder structure inaccordance with FIG. 4A. It comprises the series resonators ST1, ST2,ST3 and the parallel resonators PT1, PT2. The band-stop filter BS isembodied in one stage and has the ladder structure shown in FIG. 4C. Itcomprises the series resonator SB1 and the parallel resonator PB1. Thereception filter RX consists of a chain circuit of two ladder structuresin accordance with FIG. 4D and one ladder structure in accordance withFIG. 4B. It comprises the series resonators SR1, SR2 and the parallelresonators PR1, PR2 and PR3. The inductance L12 shifts the resonantfrequency of PB1. In addition, it is possible to provide matchingelements at the terminals LNA and PA. If it is taken into account thatthe matching circuit M2 is not present, the arrangement corresponds tothe duplexer D shown in FIG. 1.

In one exemplary embodiment, the parallel resonators and seriesresonators are bulk acoustic wave (BAW) resonators. Precisely atrelatively high frequencies of approximately 2 GHz, BAW resonators havebetter electrical properties given the same dimensions by comparisonwith surface acoustic wave (SAW) resonators. They substantially consistof a piezoelectric layer that is arranged between two electrodes andtogether oscillate at a resonant frequency. In the case of BAWresonators, the frequency of the antiresonance lies above the frequencyof the resonance. Alternatively, the resonators can also be GBAWresonators.

The parallel resonators have a lower resonant frequency than the seriesresonators. Since they lie in the passband path, the series resonators,by virtue of their antiresonance, form the upper edge of the passband ofthe transmission filter and of the reception filter, while the parallelresonators dissipate to ground and, by virtue of their resonance, formthe lower edge. The bandwidth can be set by means of the differences inthe frequencies.

In the case of BAW manufacturing technology, only two resonances fs andfp are usually available on one chip. More than two resonant frequenciesare possible, but at the cost of higher process outlay and hence a lowercomponent yield. Since the transmission filter TX and the receptionfilter RX have different center frequencies, they are thereforemanufactured on dedicated substrates CT and CR.

In FIG. 5, the band-stop filter BS is integrated together with thetransmission filter TX on the same substrate CT. The resonators of theband-stop filter BS are manufactured by means of the same manufacturingsteps as the resonators of the transmission filter TX. The band-stopfilter and the transmission filter therefore have the same resonators.In this way, no additional substrate is required for the band-stopfilter BS and additional process steps are obviated.

In order to achieve a band-stop filter BS in the passband of thetransmission filter TX with the same resonators, however, it isnecessary to arrange the resonators differently: the parallel resonatorPB1 of the band-stop filter BS is therefore the same resonator as theseries resonators ST1, ST2 or ST3 of the transmission filter TX anddissipates to ground GND via the inductance L12 at the same resonantfrequency. The series resonator SB1 of the band-stop filter BS is thesame resonator as a parallel resonator PT1, PT2 of the transmissionfilter TX. By virtue of the fact that the same resonators are used forthe band-stop filter BS and the transmission filter TX, somerestrictions arise in the design of the duplexer, but, as describedlater, they can be circumvented.

While the band-stop filter BS in FIG. 5 is integrated together with thetransmission filter TX on one substrate CT, thus resulting inconsiderable advantages during manufacture, the band-stop filter BS canalso be manufactured by means of dedicated components externally or in amanner integrated in the housing.

The resonators SB1, PB1 of the band-stop filter BS could also berealized on the substrate CR of the reception filter RX. However, thisnecessitates a more complex BAW manufacturing process that makes itpossible to be able to produce a third and, if appropriate, furtherresonator types having different resonant frequencies on one chip.

It is advantageous if in the arrangement, in contrast to theillustration in FIG. 5, the transmission filter begins with a seriesresonator, or a series resonator SR is arranged between the band-stopfilter BS and the first parallel resonator PT1 of the transmissionfilter.

One essential advantage of the band-stop filter BS manufactured with BAWresonators compared with the use of external inductances which arearranged in the housing, in the duplexer module or on the circuit boardis that the inductance values are subjected to great fluctuationsrelative to the frequency stability and manufacturing stability of theresonators. The transmission frequency isolation which can be specifiedas typical is therefore impaired unnecessarily. By contrast, thebandpass filter BS shown in FIG. 5 is constructed with acousticcomponents which, firstly, are frequency-trimmed and, secondly, are verythermally stable at approximately −20 ppm/K in the case of BAWresonators. The typical isolation can therefore be specified withsmaller allowances, as a result of which requirements made of themanufacturing process can be reduced or the yield increases in the samemanufacturing process.

The resonators of the reception filter RX can also be manufactured withSAW resonators instead of BAW resonators. SAW resonators have theadvantage that they additionally afford a matching of “single ended” to“balanced” and possibly have better electrical properties. Thecombination of SAW technology and BAW technology forms a hybridduplexer.

It is possible here to realize the transmission filter TX, the band-stopfilter BS and the reception filter RX with SAW resonators. In this case,the SAW elements can be realized on a single substrate since, in thecase of SAW manufacturing technology, resonators having differentresonant frequencies are realized without relatively great outlay, e.g.,by choosing suitable finger periods of the interdigital transducers(IDT).

In order to alter the resonant frequency of the resonators of theband-stop filter BS in order thus to optimize the electrical properties,there are a number of possibilities. In the case of a BAW resonator,additional mass coatings can be applied, which increase the oscillationmass of the resonator. One possibility consists in applying a materiallayer that can also be partly removed. The resonators with the masscoating then have a lower resonant frequency than those which have noadditional material layer or in which the material layer is only partlypresent.

While the band-stop filter BS in FIG. 5 is integrated together with thetransmission filter TX on one substrate CT, thus resulting inconsiderable advantages during manufacture, the band-stop filter BS canalso be manufactured by means of dedicated components externally or in amanner integrated in the housing.

FIG. 6 shows exemplary embodiments in which a resonator comprises aplurality of resonators, thus giving rise to further degrees of freedomfor the duplexer design. In comparison with FIGS. 4A and 4C, the seriesresonator S is replaced by the series resonators S1 and S2 in FIGS. 6Aand 6B. FIG. 6C shows the replacement of the parallel resonator P fromFIG. 4C by the parallel resonators P1 and P2. In comparison with FIG.4C, the resonators S, P have been replaced by the series resonators S1,S2 and the parallel resonators P1 and P2 in FIG. 6D. In order that thereplacing resonators have the same static capacitance C0 as the originalresonators S, P, these must have double the capacitance, i.e., doublethe area, if they are connected in series and half the capacitance,i.e., half the area, if they are connected in parallel. In one of theresonators that are present doubly, the resonant frequency can bealtered by up to 3% relative to the original resonators S, P, asdescribed above. With the additional resonant frequency, furtherpossibilities for optimization arise without the introduction of a newmanufacturing process for BAW resonators with further resonantfrequencies.

FIG. 7 shows an exemplary embodiment with a band-stop filter BS having afirst terminal 1 and a second terminal 2, and with a matching networkhaving a first inductance L1 and a second inductance L2. The band-stopfilter BS consists of an individual series resonator SB1 and can againbe manufactured together with the resonators of the transmission filterTX on the same substrate CT by means of the same process steps. L1 andL2 and the static capacitance of SB1 are designed such that uponreception of a reception signal the transmission filter TX constitutesan open circuit from the point of view of the reception filter RX andreflections between the reception filter RX and the antenna terminal ANTare minimized. Furthermore, upon transmission of a transmission signalthe reception filter RX is intended to constitute an open circuit fromthe point of view of the transmission filter TX and reflections betweenthe transmission filter TX and the antenna terminal ANT are intended tobe minimized. In this case, the first inductance L1 serves for matchingthe transmission filter TX to the antenna terminal ANT. Together withthe static capacitance C0 of the series resonator SB1 and with thesecond inductance L2, the first inductance L1 serves to form a PInetwork used for matching the reception filter RX. The matching circuitfurthermore comprises the inductances L3 and L4. In this case, theinductances L3 and L4 serve for matching the transmission filter TX andthe reception filter RX to the transmission amplifier terminal PA andthe reception amplifier terminal LNA. In this case, the transmissionfilter TX and the reception filter RX can be configured in any desiredmanner. The matching can also be effected differently, e.g., by means ofparallel inductances or a network having a predominantly inductivecharacter.

The band-stop filter BS is intended to have a high-impedance behavior attransmission frequencies, i.e., to form an open circuit, as a result ofwhich easier matching is possible. For this purpose, the first seriesresonator SB1 has an antiresonance frequency in the range of thepassband TP of the transmission filter TX. Consequently, the receptionfilter RX always constitutes an open circuit from the point of view ofthe transmission filter TX. The use of a leading parallel resonator isnot possible at this location since this would lead to a short circuitin the transmission frequency range. The series resonator SB1 can be aparallel resonator of the transmission filter TX, the resonant frequencyof which is optionally lowered.

The resonance of the series resonator SB1 lies up to 3% below thetransmission filter passband side in terms of frequency, such that noimpairment of the left transmission filter edge occurs. This can be madepossible by a lowering of the resonant frequency of the series resonatorSB1 using the means mentioned above. Generally, it is not absolutelynecessary in this case for the antiresonance to lie exactly in thecenter of the passband.

A further solution to this problem is the use of an inductance connectedin series with the band-stop filter BS. In this way, the resonance ofthe band-stop filter BS, with the same antiresonance position, is pulledto lower frequencies and the left passband edge of the transmissionfilter TX remains unimpaired. At frequencies of approximately 2 GHz,however, the series inductance would have to be greater than 10 nH, as aresult of which this solution is restricted to applications at higherfrequencies.

FIG. 8 shows an extension of FIG. 7 by a parallel resonator PB1, whichis connected to the second terminal 2 and is connected by its firstterminal to ground GND. The antiresonance of the first series resonatorSB1 is again chosen such that it lies in the region of the lowerpassband filter edge. This can be effected, e.g., by means of aresonator using BAW technology, the frequency of the resonator beinglowered by means of an additional mass coating. A series resonator ofthe transmission filter TX can be chosen for the parallel resonator PB1.The matching circuit is unchanged in its structure by comparison withFIG. 7. The inductance L2 can advantageously turn out to be smallerowing to the presence of the parallel resonator PB1. A leading SB1resonator decouples the transmission filter TX from the reception filterRX, as a result of which the matching is simplified. The antiresonancelies in the passband of the transmission filter, or up to approximately3% below, and the resonance lies below the passband, in order that theleft passband edge is not impaired. Depending on the requirement made ofthe band-stop filter, even further parallel resonators can also beconnected to the second terminal 2 and to ground GND. For the matching,the first terminals of the parallel resonators can be connected toground GND at least in part also via inductances instead of a directconnection. The resonant frequencies of the further parallel resonatorscan differ from one another by up to 3%.

FIG. 9 shows a modification of FIG. 7, in which the series resonator SB1has been replaced by two series resonators SB1 and SB2 connected inseries. Instead of the matching circuit in FIG. 7, designed as a PInetwork, a T-network is used for matching the reception filter RX. Thecapacitances of the T-network are formed from the static capacitances ofthe series resonators SB1 and SB2. The inductance necessary for theT-network is supplied by the coil L2, which connects the connecting nodeA, via which the series resonators SB1 and SB2 are connected to oneanother, to ground GND. The series resonator SB1 again has the loweredresonant frequency of a parallel resonator of the transmission filterTX, such that its antiresonance lies at the lower bandpass edge of thetransmission filter TX. The first inductance L1 supplies thepredominantly inductive character at the antenna terminal ANT. Theresonator SB2 is a parallel resonator of the transmission filter.

FIG. 10 shows a further exemplary embodiment, which is based on FIG. 9.In FIGS. 7 to 9, the matching of the transmission filter TX is notindependent of the matching of the reception filter RX. Thisdisadvantage can be avoided with the matching circuit shown in FIG. 10.By comparison with FIG. 9, the first inductance L1 no longer connectsthe first terminal 1 to the antenna terminal ANT, but rather to thetransmission filter TX. However, the attainment of the open-circuitcondition for the reception filter RX is more difficult with thearrangement shown. It is helpful if the transmission filter TX has aseries resonator, such as, for example, the ST3 in FIG. 5, via which itis connected to the first inductance. The antenna terminal ANT isopen-circuited in the reception frequency range.

FIG. 11 shows an exemplary embodiment in which a plurality of filtersTX1, TX2, TX3, RX1, RX2 are coupled to a common node K. Such anarrangement is present, e.g., in module applications in which aplurality of transmission and reception paths exist. A high isolationbetween each transmission path and each reception path, as alreadydescribed above, is desired here. Furthermore, the intention is also tominimize capacitive losses that arise by virtue of the fact thatbandpass filters having a sufficiently large band gap with respect toone another act on one another like a capacitive loading if they areconnected to the common node K. The isolation between transmission andreception paths can be achieved in the manner described above. In orderto reduce the insertion loss of the bandpass filter TX1 as a result ofcapacitive losses on account of the bandpass filters TX2 and TX3, aband-stop filter BS is connected upstream thereof. The band-stop filterBS can be a resonator whose antiresonance lies approximately in thebandpass center of the first bandpass filter TX1. The capacitive loadingby the further bandpass filters RX1 and RX2 can also be eliminated inthe same way. The matching circuits required are not shown in FIG. 11.All the abovementioned steps for the matching and selection of theresonators for the band-stop filter BS and also the variations describedare also applicable to FIGS. 7 to 11.

1. A duplexer, comprising: an antenna terminal; a transmission amplifierterminal; a reception amplifier terminal; a transmission filter, whereinthe transmission amplifier terminal is coupled to the antenna terminalvia the transmission filter, wherein the transmission filter has apassband, and wherein the transmission filter comprises at least oneresonator; a reception filter coupled to the reception amplifierterminal; and a band-stop filter, wherein the reception filter iscoupled to the antenna terminal via the band-stop filter, wherein theband-stop filter has a stop band in the passband of the transmissionfilter, and wherein the band-stop filter comprises at least one sameresonator as the transmission filter.
 2. The duplexer according to claim1, wherein the transmission filter and the band-stop filter compriseladder structures having series resonators or parallel resonators. 3.The duplexer according to claim 2, wherein the transmission filter hasat least one parallel resonator and a plurality of series resonators andwherein at least one parallel resonator of the transmission filter has aresonant frequency that is lower than resonant frequencies of the seriesresonators of the transmission filter.
 4. The duplexer according toclaim 2, wherein at least one series resonator of the band-stop filteris the same resonator as a parallel resonator of the transmissionfilter.
 5. The duplexer according to claim 2, wherein at least oneparallel resonator of the band-stop filter is the same resonator as aseries resonator of the transmission filter.
 6. The duplexer accordingto claim 2, wherein a resonant frequency of at least one seriesresonator of the band-stop filter differs from resonant frequencies ofthe remaining series resonators of the band-stop filter.
 7. The duplexeraccording to claim 4, wherein a resonant frequency of at least oneparallel resonator of the band-stop filter differs from resonantfrequencies of the remaining parallel resonators of the band-stopfilter.
 8. The duplexer according to claim 6, wherein the resonatorswhose resonant frequencies differ from the other resonators have anadditional mass coating that lowers the resonant frequency and which isnot present, or not present to the same extent, in the other resonators.9. The duplexer according to claim 8, wherein the additional masscoating comprises a passivation layer, a silicon dioxide layer, atungsten layer, a titanium layer, an aluminum layer, a molybdenum layer,an iridium layer, a ruthenium layer, a silicon nitride layer, analuminum nitride layer, a zinc oxide layer, a lead zirconate titanatelayer or a barium strontium titanate layer, and the layer is appliedabove or below a piezo-layer of the resonator, or thickens thepiezo-layer.
 10. The duplexer according to claim 1, wherein theresonators of the transmission filter and the resonators of theband-stop filter are BAW resonators and are arranged on the samesubstrate.
 11. The duplexer according to claim 1, wherein the resonatorsof the reception filter and the resonators of the band-stop filter areBAW resonators and are arranged on the same substrate.
 12. The duplexeraccording to claim 1, wherein the resonators of the transmission filter,the resonators of the reception filter and the resonators of theband-stop filter are BAW resonators and are arranged on the samesubstrate.
 13. The duplexer according to claim 12, wherein the receptionfilter comprises at least one SAW resonator.
 14. The duplexer accordingto claim 1, wherein the resonators of the transmission filter and theresonators of the band-stop filter are SAW resonators and are arrangedon the same substrate, and wherein the SAW resonators of the receptionfilter, of the transmission filter and of the band-stop filter areconstructed on a common substrate.
 15. The duplexer according to claim1, wherein the at least one resonator of the transmission filtercomprises a GBAW resonator.
 16. The duplexer according to claim 1,wherein at least one of the resonators of the band-stop filter has afirst resonant frequency and a first static capacitance, and comprisesat least a number N of resonators, wherein the number N is greater thanor equal to two, the N resonators each have a static capacitance that isN times greater than the first static capacitance, the N resonators areconnected in series with one another, and the resonant frequencydeviates from the first resonant frequency by up to 3% at least in oneof the N resonators.
 17. The duplexer according to claim 1, wherein atleast one of the resonators of the band-stop filter has a first resonantfrequency and a first static capacitance, and comprises at least anumber N of resonators, wherein the number N is greater than or equal totwo, the N resonators each have a static capacitance that is N timesless than the first static capacitance, the N resonators are connectedin parallel with one another, and the resonant frequency deviates fromthe first resonant frequency by up to 3% at least in one of the Nresonators.
 18. The duplexer according to claim 1, further comprising atleast one matching circuit designed such that upon reception of areception signal; wherein the transmission filter constitutes an opencircuit from the point of view of the reception filter; whereinreflections between the reception filter and the antenna terminal areminimized, and upon transmission of a transmission signal; wherein thereception filter constitutes an open circuit from the point of view ofthe transmission filter; and wherein reflections between thetransmission filter and the antenna terminal are minimized.
 19. Theduplexer according to claim 18, wherein the band-stop filter has a firstterminal and a second terminal, wherein the first terminal is connectedto the antenna terminal and the transmission filter and the secondterminal is connected to the reception filter, the matching circuitcomprises a first inductance and a second inductance, wherein the firstinductance connects the first terminal to ground and the secondinductance connects the second terminal to ground.
 20. The duplexeraccording to claim 19, wherein the band-stop filter comprises a firstseries resonator that is connected to the first terminal.
 21. Theduplexer according to claim 20, wherein the first series resonator hasan antiresonance lying in the passband of the transmission filter or upto three percent below that in terms of frequency.
 22. The duplexeraccording to claim 20, wherein the band-stop filter further comprises atleast one parallel resonator, wherein a first terminal of the at leastone parallel resonator is connected to ground, and the other terminal ofthe at least one parallel resonator is connected to the second terminaland the first series resonator.
 23. The duplexer according to claim 22,wherein at least one of the first terminals of the at least one parallelresonator is connected to ground not directly but rather via aninductance, or a capacitance, or a combination of an inductance and acapacitance.
 24. The duplexer according to claim 23, wherein the firstseries resonator has an antiresonance in the region of the lowerpassband edge of the transmission filter and the parallel resonator hasa resonant frequency lying in the region of the center of the passbandof the transmission filter.
 25. The duplexer according to claim 20,wherein the first series resonator is the same as a parallel resonatorof the transmission filter, but has an additional mass coating thatreduces the resonant frequency of the first series resonator relative tothe resonant frequency of the parallel resonator, and the parallelresonator connected to the second terminal is the same as a seriesresonator of the transmission filter.
 26. The duplexer according toclaim 18, wherein the band-stop filter comprises a first terminal and asecond terminal, and two series resonators connected in series via aconnecting node, of which one series resonators is connected to thefirst terminal and the other series resonator is connected to the secondterminal, the first terminal is connected to the transmission filter andthe second terminal is connected to the reception filter, and thematching circuit comprises a first inductance and a second inductance,wherein the first inductance connects the first terminal to the antennaterminal and the second inductance connects the connecting node toground.
 27. The duplexer according to claim 18, wherein the band-stopfilter comprises a first terminal and a second terminal, and two seriesresonators connected in series via a connecting node, of which oneseries resonators is connected to the first terminal and the otherseries resonator is connected to the second terminal, the first terminalis connected to the antenna terminal, and the first inductance does notconnect the first terminal to the antenna terminal, but rather to thetransmission filter, the matching circuit comprises a first inductanceand a second inductance, wherein the first inductance connects the firstterminal to the transmission filter and the second inductance connectsthe connecting node to ground.
 28. The duplexer according to claim 26,wherein the series resonator connected to the first terminal has anantiresonance in the region of the lower passband of the transmissionfilter, and the series resonator connected to the second terminal has anantiresonance frequency lying in the region of the center of thepassband of the transmission filter.
 29. The duplexer according to claim26, wherein the series resonator connected to the first terminal is thesame as a parallel resonator of the transmission filter, but has anadditional mass coating that reduces the resonant frequency of theseries resonator relative to the resonant frequency of the parallelresonator, and the series resonator connected to the second terminal isthe same as a parallel resonator of the transmission filter.
 30. Theduplexer according to claim 20, wherein the first series resonator hasan antiresonance but has no resonance.
 31. The duplexer according toclaim 20, wherein at least one parallel resonator has a resonance buthas no antiresonance.
 32. The duplexer according to claim 29, whereinthe transmission filter comprises a series resonator, the transmissionfilter being connected to the first terminal via the series resonator.33. The duplexer according to claim 18, wherein the matching circuitfurther comprises: an inductance that connects the transmission filterto the transmission amplifier terminal; and an inductance that connectsthe reception filter to the reception amplifier terminal.
 34. Theduplexer according to claim 1, further comprising further terminals thatare coupled to the antenna terminal via respective filters and band-stopfilters, wherein the stop bands of the respective band-stop filters liein the passband of the transmission filter.
 35. A circuit comprising: afirst bandpass filter, a band-stop filter; and at least one secondbandpass filter, wherein the first bandpass filter and the at least onesecond bandpass filter are coupled to a common node, wherein the atleast one second bandpass filter is coupled to the common node via theband-stop filter, wherein the band-stop filter effects a suppression ina passband of the first bandpass filter, and wherein a filter functionof the first bandpass filter and a filter function of the band-stopfilters are each realized by at least one same resonator.
 36. Thecircuit according to claim 35, wherein resonators of the first bandpassfilter and resonators of the at least one band-stop filter are realizedon the same substrate.
 37. The circuit according to claim 36, whereinthe same resonators are realized by means of the same process steps. 38.The circuit according to claim 35, wherein the resonant frequency of atleast one resonator of the band-stop filters is lowered relative to theresonant frequency of the same resonator of the first bandpass filter.39. The circuit according to claim 38, wherein the resonant frequency islowered by an additional mass coating applied on the resonator.
 40. Thecircuit according to claim 35, wherein resonators of the band-stopfilters and of the first bandpass filter are BAW resonators.
 41. Thecircuit according to claim 35, wherein resonators of the band-stopfilters are SAW resonators.
 42. The circuit according to claim 35,wherein resonators of the band-stop filters are GBAW resonators.
 43. Thecircuit according to claim 35, further comprising an antenna coupled tothe common node wherein the first bandpass filter, the at least onesecond bandpass filter and the antenna are impedance-matched to oneanother such that at frequencies lying in the passband of the firstbandpass filter, wherein the reflection of power between the firstbandpass filter and the antenna is minimized, and wherein the at leastone second bandpass filter constitutes an open circuit from the point ofview of the first bandpass filter, and at frequencies lying in each casein the passbands of the at least one second bandpass filter, wherein thereflection of power between each second bandpass filter and the antennais minimized, and wherein the first bandpass filter constitutes an opencircuit in each case from the point of view of the respective secondbandpass filter.
 44. The circuit according to claim 43, wherein a staticcapacitance of at least one resonator of the band-stop filters is variedfor the matching.
 45. The circuit according to claim 43, wherein atleast one of the resonators of the band-stop filters has a firstresonant frequency and a first static capacitance and is realized by atleast a number N of resonators, wherein the number N of resonators isgreater than or equal to two, the N resonators each have a staticcapacitance that is N times greater than the first static capacitance,the N resonators are connected in series with one another, and theresonant frequency deviates from the first resonant frequency by up to3% at least in one of the resonators.
 46. The method according to claim43, wherein at least one of the resonators of the band-stop filters hasa first resonant frequency and a first static capacitance and isrealized by at least a number N of resonators, wherein the number N ofresonators is greater than or equal to two, the N resonators each have astatic capacitance that is N times less than the first staticcapacitance, the N resonators are connected in parallel with oneanother, and the resonant frequency deviates from the first resonantfrequency by up to 3% at least in one of the resonators.